Well Silver Surfer rights most likely are stuck with Fox, but supposedly Namor is not part of the FF agreement (or at least, that's what some on this board have said). If true, Strange, Hulk, and Namor are essential. I think Nighthawk and Valkyrie would be a nice way to round out the team, but the movie may be better served just playing the main (available) three against each other, considering that none of them really get along very well with others for different reasons. Namor should just constantly insult the other guys all movie long, and should knock the **** out of Bruce Banner at some point.
Ohhh. That'd be a good one. Who else would you like to see on that team?